In semiconductor chips (hereinafter abbreviated as “chips”) such as memory LSI (Large Scale Integrated circuits) and logic LSI that are formed on a silicon substrate and diced, electrodes of the chip must be electrically connected to wirings of a printed wiring board. According to a prior art technology, the chip is connected to a lead frame as is then sealed, and a casing in which the chip is sealed (hereinafter called “chip package”) is connected to the printed wiring board.
As performance of the chip has become higher in recent years, the number of pins of the bonding pad and the lead frame has become greater. Therefore, in case the wire is bonded one by one to a plurality of bonding pads, a bonding time gets elongated in proportion to the number of pads. As the number of pins of the chip has become greater, a pin pitch has become narrower, and it has become more difficult to connect the lead frame to the printed wiring board without short-circuiting the lead frame.
Therefore, a free chip connection system that forms bumps to the pads on the chip side and mounts the chip to a connection substrate through the bumps has been used remarkably. Because integration density and function of LSI have become higher recently, a tendency of stacking chips and wires on a substrate into a multi-layered construction has become stronger.
Ceramics are generally used as a packaging substrate material of the semiconductor chips the integration density and function of which have become higher, but the ceramics are not free from the problems such as low planarity and high cost. Because it has become necessary to further reduce the thickness and the size, a packaging substrate material having wirings that are finer than ever has become necessary.
To solve the problems described above, glass that has higher planarity and lower cost than the ceramics has drawn an attention as the packaging substrate material. A glass substrate is excellent in high frequency characteristics and is expected to be mounted to mobile communication devices. To mount the glass substrate to mobile devices, smaller size and higher performance are required, too. A proposal had been made in which a plurality of through-holes is formed in a glass substrate having excellent planarity and smoothness and a conductor is formed in each through-hole so as to make wiring from both upper and lower surfaces of the glass substrate.
Methods of forming through-holes in a wiring glass substrate include the one that uses a photosensitive glass substrate capable of being chemically cut as described in JP-A-9-321184. According to this method, a mask for forming holes is disposed on a photosensitive glass substrate capable of being chemically cut and an exposure and development process is conducted to form through-holes for forming conductors. Via electrodes are formed in the glass substrate having the through-holes formed therein by using sputtering in combination with plating. This method accomplishes higher density wiring and smaller size than ever.
When electrodes are formed inside via-holes formed in a packaging substrate by a prior art method, the following problems may develop in the method that uses a conductive paste and in the method that uses sputtering in combination with plating.
When the conductive paste is used, the conductive paste is applied to the packaging substrate in accordance with a printing mask pattern by using a printing unit. In the case of the glass substrate, however, the temperature required for the production is lower by 200 to 300° C. than the ceramic substrate, and volume shrinkage is greater because the conductive paste is of a baking type. Therefore, it is difficult in some cases to form the electrode inside the via-hole when the conductive paste of the prior art is used.
When sputtering and plating are employed in combination, a foundation electrode is formed by sputtering on an inner wall surface of the via-hole and an electrode is deposited by plating to the foundation electrode as a base. When the foundation electrode is formed on the inner wall surface of the via-hole of the packaging substrate, however, sputtering must be carried out from both upper and lower surface sides of the substrate and the electrode formation is troublesome and time-consuming. The electrode formed by plating inside the via-hole does not always have high adhesion with the glass.
The prior art technology has not taken sufficiently the strength of the glass substrate into consideration. The strength of the substrate drops as a plurality of through-holes is formed. When the thickness of the substrate is increased to suppress such a drop, the package cannot be rendered compact and the time required for forming the through-holes becomes longer. A through-hole substrate having sufficient strength can reduce the thickness and the size and will eventually reduce the production cost.
It is an object of the invention to provide a wiring glass substrate, its manufacturing method, a conductive paste and a semiconductor module used for the wiring glass substrate, and a method of forming a wiring substrate and a conductor that have high mechanical strength, can reduce a production time and can accomplish low cost.
The above and other objects and novel features of the invention will become more apparent from the following description of the specification and from the accompanying drawings.